The present invention relates to a thermal printing apparatus having a thermal print head provided with a line buffer. More particularly, the invention relates to a thermal printing apparatus which employs a thermal print head and line buffer in order to reduce the amount of hardware, while still achieving high speed printing.
As is well known, a thermal print head converts electrical signals into thermal energy in order to print by sublimating dye. A video printing apparatus (also known as a "color image printer") prints using this type of a thermal print head (TPH). For instance, a video printing apparatus sublimates dye of the dye-deposited film by the heat-energy generated by the thermal print head, which is energized by applying current thereto. The print head then prints the desired image or picture depending on the amount of the dye printed on the recording paper.
The conventional sublimate-type thermal printing apparatus which prints using a TPH is shown in FIG. 1. The apparatus includes a system controller ("syscon") 1 for controlling the overall operation of the system. The apparatus receives image signals from a signal source, such as a video camera, television, personal computer, graphic computer, etc. The red (R), green (G), and blue (B) signals input from the signal source are stored in units of frames in a frame memory 3 under the control of a memory controller 2, which controls the timing for reading and writing of data.
A selector 4 selects the R, G, and B signals that are stored in the frame memory 3 one by one, under the control of the syscon 1. A color converter 5 converts the selected signal into a complementary color; the selected B signal is converted into a yellow (Y) signal, the selected G signal into a magenta (M) signal, and the selected R signal into a cyan (C) signal, respectively. A corrector 6 corrects the output of the color converter 5 using such methods as gamma correction, color correction, resistance correction, and a temperature correction. The corrected signals are then stored in a line memory 7.
Meanwhile, the image data that is stored in the line memory 7 and the gradation data generated from a gradation counter 8 are compared by a gradation comparator 9 for printing. In addition, if the image data read from line memory 7 is larger than the gradation data of gradation counter 8, a "1" is transmitted to the TPH 13; otherwise, a "0" is transmitted to the TPH 13, according to a clock signal generated from a clock signal generator 10.
Next, the data corresponding to the amount of one line is transmitted to the TPH 13, which then is latched by a latch signal LATCH generated from a latch signal generator 11. The latched data is then printed according to the strobe signal STROBE, which is generated from a strobe signal generator 12 in order to enable each heating element.
As shown in FIG. 2, which is a detailed circuit diagram of the TPH 13, the data whose gradation is compared in the gradation comparator 9, as described above, is stored in a shift register 14 by one bit according to the clock signal. When the data corresponding to one line amount is stored in the shift register 14, the data is output of the shift register 14 and is stored into a latch register 15 according to the latch signal. Here, the number of the heating elements for heating one line equals 512, as an example. The heating element 16, which is composed of resistors RO-R511, is electrified and emits heat depending on the two inputs to the NAND gates G0-G511; the two inputs including the strobe signal generated from the strobe signal generator 12 and the data stored in latch register 15.
Thus, when the expression of one gradation is finished, the signal read from the line memory 7 is compared with the next gradation value of the gradation counter 8 through the gradation comparator 9 in order to express the next gradation. The output of the gradation comparator 9 is transferred to the TPH 13, and after transferring one line of data, the transferred data is latched in the latch register. Then, the predetermined gradation (here, 256 gradations) is expressed during the strobe signal.
Thus, if the image data is transferred to the TPH 13, which has 512 heating elements, by inputting single-bit data, the size of the block of the TPH 13 is 512. Therefore, if the clock frequency of the TPH 13 is 5 MHz, it will take 102.4 microseconds for transferring one data. Accordingly, it takes about 26 milliseconds (102.4 times 255) for emitting heat for printing one line of zero to 255 levels, i.e., 256 gradations.
Accordingly, since the time of at least 102.4 microseconds is required for expressing one gradation, it is impossible to reduce the amount of time for printing one line down to 26 milliseconds.
In addition, the pixel data stored in the frame memory 3 is transmitted to the line memory 7 one line at a time when the conventional thermal printing apparatus as shown in FIG. 1 is provided with a thermal print head as shown in FIG. 2. Further, the transmitted data of one line is gradation-compared by the gradation comparator 9, which then is transmitted to the TPH 13 so as to repeat the expression when printing up to the 255th gradation. At this time, the quantity of hardware becomes large since the line memory 7 is necessary for compensating for the difference between the transfer speed in which the pixel data is transferred to the heating elements 16 within the TPH 13 and the data reading speed of frame memory 3.